P-type ZnO Research Articles

Self-compensation reduction as first step of p-type ZnO synthesis

Abstract In this work, we used a new strategy, based on the reduction of self-compensation, to synthesize p-type ZnO thin films. This new protocol is based on the control of intrinsic defects especially the vacancies and the interstitials of both zinc and oxygen networks. It consists of using zinc nitrate as a precursor and varying its concentration on the one hand, and the addition of acetic acid on the other. Hall Effect measurements have shown a decrease in electron concentration as the nitrate concentration increases. A concrete transition in the conductivity of ZnO from n-type to p-type has been observed for concentration around 0,4M. Thermal Analyses (DTA-TGA), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), UV–visible spectrophotometry were used to study the structural, morphological and optical properties of the synthesized layers. All these measures confirm the appearance of a change in behavior when nitrate concentration is approaching 0,4M. This transition in conductivity-type in ZnO layers has been linked to a significant increase in the oxygen content as confirmed by an EDX analysis during films growth. The growth of p-type ZnO films based on zinc nitrate can potentially lead to the development of new transparent devices applicable in optoelectronics and photovoltaic fields.

Effect of doping concentration on optical and electrical properties of intrinsic n-type ZnO (i-ZnO) and (Cu, Na and K) doped p-type ZnO thin films grown by chemical bath deposition method

Effect of doping concentration on optical and electrical properties of intrinsic n-type ZnO (i-ZnO) and (Cu, Na and K) doped p-type ZnO thin films grown by chemical bath deposition method

Non-axial NO-VZn shallow acceptor complexes in nitrogen implanted p-type ZnO thin films

Over the past two decades, there have been numerous reports about the p-type behavior of N-doped ZnO. To date, however, its origin still remains mysterious, especially, N ion-implanted ZnO system. Herein, ZnO films were implanted with 70 keV N ions to a fluence of 1 × 1017 cm−2 at room temperature, followed by annealing in the range of 750–950 °C. The obtained p-type ZnO films have a widely hole concentration of 2.87 × 1015 ~ 2.64 × 1016 cm−3, a mobility of 1.37 ~ 7.27 cm2V-1s−1 and a resistivity of 148.3 ~ 299.4 Ω.cm. The thermal evolution of point defects and the possible shallow acceptors in N-implanted ZnO films were further investigated by means of Raman scattering, Photoluminescence (PL) and Electron paramagnetic resonance (EPR). The results show that abundant intrinsic-related defects including zinc interstitials (Zni), oxygen vacancies (VO) and zinc vacancies (VZn) were introduced during ion implantation. It is demonstrated that appropriate post-annealing can not only reduce the compensation of donor defects, but also facilitate the formation of N-related shallow acceptor complexes, both of which contribute to the p-type conduction transition of N-implanted ZnO films. The non-axial NO-VZn complexes are proposed to be a kind of potential and stable acceptors in N ion-implanted ZnO films.

Film thickness effect in c-axis oxygen vacancy-passivated ZnO prepared via atomic layer deposition by using H2O2

The presence of oxygen vacancies in zinc oxide is a long-standing problem preventing the fabrication of p-type ZnO materials; however, the use of H2O2 as the oxygen source for preparing ZnO films via atomic layer deposition can significantly suppress their formation. Thus, this paper presents the investigation of the film thickness effect on the properties of oxygen vacancy-passivated ZnO prepared using H2O2. Increasing the ZnO film thickness increased the crystallinity along the c-axis preferred growth orientation, the grain size, and the surface roughness but also reduced the c-axis tensile strain, oxygen vacancies, optical bandgap, and film transmittance. The oxygen vacancy decrease was due to the reduced grain boundaries and improved crystallinity resulting from the film thickness increase, consequently weakening the resistivity through a large decrease in carrier concentration and a small increase in carrier mobility.

(Li,F) co-doped ZnO: Optoelectronic devices applications

Abstract One of the challenging tasks of ZnO is to make its p-type doping stable. However, the physical mechanism behind the experimental phenomena of stable ZnO p-type has not been explored so far. Here, the first-principles calculations using the GPW and FP-LPAW methods within Density Functional Theory (DFT) were investigated to explore, on one side, the effect of different substitutional and interstitial sites of Li insertion, and on the other side, the effect of anion-cation co-doping (Li–F) at ratios of 1:1 and 2:1, respectively, on the structural stability as well as electronic, optical, and electrical properties of ZnO. The results showed that Li at the octahedral site (Lioct) acting as donor-type is more stable than Li at the Zn site (LiZn) acting as acceptor-type, explaining the difficulty to obtain a stable ZnO p-type conductivity. For equal proportion co-doping under O-rich condition, the formation energy of LiZn–F is lower than that of Lioct-F, indicating that the (LiZn–F) can suppress the effect of Li interstitial and form a completely passive complex. Furthermore, the formation energy of (2LiZn–F) co-doping was more reduced compared to that of (LiZn–F) co-doping with the formation of an occupied shallower acceptor level that improves holes electrical conductivity. Also, a significant absorption in the visible region was shown compared to pure ZnO. Therefore, we can conclude that the presence of the F atom could suppress the formation of interstitial, and (2Li,F) co-doping can be a promising stable p-type of ZnO for use in optoelectronic device applications.

A simple and effective approach to fabricate transparent p-n homojunction KZO/ZnO thin films

Transparent p-type potassium-doped ZnO (KZO) thin films are fabricated by a simple and effective approach of molten-salt treatment (MST) in KNO3 for ZnO thin films, to further fabricating p-n homojunction KZO/ZnO thin films with one-step method. XRD and PL results show the influence of MST on ZnO crystal, especially on the grain size. The transmittance of KZO/ZnO thin films has been slightly improved in visible-light region. Notably, after MST at 400 °C, hall-effect test reveals that the carrier type successfully transfers to p-type, and I-V curve hints that the p-n homojunction KZO/ZnO could be considered formed. Meanwhile, TEM, SIMS and XPS results reveal that K has doped into KZO thin films, resulting in the generated p-type carriers.

Effect of multiple-step annealing on the structural, optical and electrical properties of ZnO:In-N films

The intrinsic defects in ZnO:In-N films can be regulated efficiently by multiple-step annealing together with the ambient atmosphere. Correspondingly, the conductivity of films changes from n to p-type, then to n-type (n → p → n). The experimental results indicate high-quality films annealed at 600 °C for 8 min in N2 ambience can be obtained by the analysis of X-ray diffraction (XRD) patterns and the concentrations of donor defects (Zni, VO) are dramatically decreased from Raman and photoluminescence (PL) spectra. When the films are further annealed in air for 3 min, the concentrations of acceptors such as oxygen interstitials (Oi) are increased slightly. Meanwhile, N-related acceptor complexes in the films are dominantly activated in terms of relative X-ray photoelectron spectroscopy (XPS) peak area ratio of P1 (395.5 eV) and P2 (397.3 eV) to P3 (403.5 eV), which plays an important role in realizing the conversion of the ZnO p-type conductivity. Noted that the p-type ZnO:In-N films with the hole concentration of 1017 cm−3 can be obtained in this way.

Novel transportation layers of NiO and ZnO for the fabrication of perovskite solar cell for e-monitoring of healthcare

Inorganic n-type NiO and p-type ZnO has been used as hole transport layers (HTL) and electron transport layers (ETL), respectively, for the fabricated perovskite solar cells. The sols of NiO and ZnO have been synthesized using sol–gel method. The structural, morphological, and optical analysis of the synthesized ZnO and NiO nanoparticles prepared from sol has been done. The spin coating technique was used for the fabrication of HTL, ETL, and perovskite layers. The thickness of the fabricated solar cell is about 2 μm. The current density versus voltage (JV) characteristics of the fabricated solar cell was investigated. The JV characteristics revealed the value of various parameters such as short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), power conversion efficiency, and maximum power (Pmax) is 15.90 mA/cm2, 0.93 V, 73%, 10.70%, and 10.70 W, respectively. The fabricated solar cell has been used to design a special electronic circuit with maximum power point tracker (MPPT) for monitoring of patient suffering from the heart diseases. The performance parameters of MPPT circuit have been tested under “direct sunlight” and “artificial shadow.” The energy consumption and 1-day performance load of the designed circuit is tested with the solar cell. The e-monitoring of patient suffering from heart diseases is done with the help of bluetooth low energy (BLE) to measure blood pressure (BP), heartbeat, photoplethysmography (PPG), electrocardiogram (ECG) signals.

Effects of oxygen partial pressure on the structural and electrical properties of Al and Sb co-doped p-type ZnO thin films grown by pulsed laser deposition

ZnO:Al0.01Sb0.02 thin films were grown on the c-plane sapphire via pulsed laser deposition at 500 °C under different oxygen partial pressures (0.13–66.66 Pa) and their structural and electrical properties were investigated. The thin films deposited under low oxygen partial pressures (0.13–7.99 Pa) grew with a c-axis preferred orientation and a 30° rotated one. At higher oxygen partial pressures (13.33–66.66 Pa), in addition to the main c-axis-oriented domains, partial domains with 32°- and 90°-tilted c-axes were observed. The ZnO:Al0.01Sb0.02 thin films deposited at low oxygen partial pressure and high oxygen partial pressure exhibited n-type and p-type conduction, respectively. The X-ray photoelectron spectroscopy analysis exhibited that the thin films contained more Sb5+ than Sb3+; therefore, the p-type conduction was owing to the Sb5+ ions, which formed the SbZn–2VZn complex.

Effect of biaxial strain on the p-type of conductive properties of (S, Se, Te) and 2 N co-doped ZnO

Although the physical properties of ZnMxO1-x-yNy (M = S, Se, Te) p-type doping have been reported, the effects of biaxial strain on (S, Se, Te) and 2 N co-doped ZnO are rarely investigated by first-principles studies. Therefore, this study employs the first-principle plane wave ultra-soft pseudopotential method based on density functional theory to explore the formation energy and conductive properties of the doping system with a co-doping ratio of M and N in ZnO of M:N = 1:2 under the strain range of ‒5 % to 5 %. Under unstrained conditions, the M-N doping position is fixed and the other N is closer to M. The closer the N-N distance, the smaller the formation energy of the same doping system, and the higher the stability of the doping system. Regardless of the tensile or compressive strain, the formation energy of the doping system is higher than that of the unstrained doping system. The stability decreases relatively, but the formation energy is negative and the stability is still high. When the compressive strain is ‒5 %, the hole conductivity of the ZnSe0.0278O0.9166N0.0556 doping system is greater than that of the unstrained ZnSe0.0278O0.9166N0.0556. The vertical and horizontal comparisons show that the p-type effect and hole conductivity of ZnSe0.0278O0.9166N0.0556 are optimal when the compressive strain is ‒5 %. This study can be used as a guide for the design and preparation of new p-type ZnO conductive functional materials.

Identification and control of defects in nitrogen-doped ZnO

ZnO is regarded as a wide band gap semiconductor with high-efficient luminescent properties due to the stable existence of excitons at room-temperature. However, the prominent opto-electronic properties have not been achieved in real application because the defects existing in the material are quite complex in terms of form and property. Also, the p-type doping of ZnO is well-known to be extremely hard. As a result, more and more recent works have shown that the properties of the ZnO material are of high-possibility to be determined by the defects in ZnO. For instance, in p-type doped ZnO, free holes could be a result of complex defects rather than the anticipated dopants. Therefore, it is quite important to investigate the forms, properties, and control of various defects in ZnO. In this article, we will discuss the properties and control of various intrinsic and extrinsic defects in ZnO by reviewing the recent progress on related studies. Firstly, the formation, form, and control technique of the zinc interstitial related donors will be discussed. It is shown that the zinc interstitial related defect is an important compensating source to holes in nitrogen-doped ZnO material. The defects could be identified and tracked quantitively by a combination of a batch of characterization methods, such as electron paramagnetic resonance, X-ray photoelectron spectroscopy, and Raman backscattering spectroscopy. The defects could be controlled by appropriately setting the condition of growth and/or post thermal treatment. Secondly, the main origin of possible acceptors in nitrogen-doped ZnO material has been pointed out. Intrinsic vacancies in combination with the extrinsic dopants, nitrogen, could be possible acceptors with shallow activation energy. Meanwhile, zinc vacancy small clusters could also be a candidate responsible for the shallow states of acceptors in ZnO. Moreover, novel complex acceptors as proposed by first-principles theoretical calculations have been experimentally introduced and realized. Under O-rich condition during nitrogen doping of ZnO, nitrogen molecule (N2) or nitrogen-hydrogen (NH x ) complexes could be incorporated on the zinc site, having shallow activation energy to contribute holes. As a result, the origin of p-typeness realized in nitrogen-doped ZnO could be partially understood. Combining the photoluminescence characterization, the corresponding relation between the defects and the optical emission lines has been established. Such a “fingerprint” could be utilized to assist identify defects in ZnO. Thirdly, we will discuss the isovalent-acceptor (IA) co-doping scheme, like tellurium-nitrogen or sulfur-nitrogen co-doping in ZnO. Such a co-doping technique could be a feasible route for obtaining high-efficient p-type ZnO material by suppressing compensation and enhancing activation rate of free holes. Zinc interstitials related donor-like complexes could be well suppressed while the co-doped isovalent element is beneficial to stabilizing nitrogen, restoring strain mismatch, and elevating the valence band maximum (VBM). The elevated VBM can further facilitate the ionization of acceptors, making the p-typeness much prominent. At last, using the IA co-doping technique, we will show the realization of ZnO homojunction light-emitting diode made by IA-co-doped ZnO nano-rod on high-quality ZnO thin films at room temperature and other applications of ZnO material to devices, like high-performance non-volatile memory and broadband photo-detector. The current problem, perspective and outlook of the defects and p-type doping in ZnO have been briefly discussed at the end of the article.

Cation impurity-defect complex induced ferromagnetism and hopping conduction in Sb-doped ZnO synthesized under high pressure

Sb-doped p-type ZnO was synthesized using the high-pressure method with a quasi-equilibrium thermodynamic process. Room temperature ferromagnetism was observed in Sb-doped ZnO. The ferromagnetic coupling was found to be associated with zinc vacancies (VZn) induced by Sb doping, which was supported by a strong ultraviolet emission peak relevant to VZn in low temperature photoluminescence. Hall effect measurements indicated that the Sb-doped ZnO with Sb content of 3.4 at% had a hole concentration of 1.4 × 1020 cm−3 and low resistivity of 7.4 × 10−2 Ω cm. Temperature-dependent resistance measurements suggest that the electrical conduction mechanism of the Sb-doped ZnO changed from thermal activation model and nearest-neighbor hopping (NNH) conduction at high temperature to Mott variable-range hopping (VRH) and Efros–Shklovskii VRH conduction at low temperature. First-principles calculations demonstrated that the Sb impurity substituting Zn (SbZn) combines two VZn defects to form acceptor-like impurity-defect complex of SbZn–2VZn in ZnO due to the coulomb attraction. The SbZn–2VZn complex in Sb-doped ZnO are strongly correlated with ferromagnetic stability and lead to the Mott and Efro–Shklovskii VRH conduction at low temperature due to the localization of carriers at the complex impurity band.

Co-doped p-type ZnO:Al-N Thin Films Grown by RF-Magnetron Sputtering at Room Temperature

This study reports the structural properties of zinc oxide thin films co-doped with aluminum and nitrogen (ZnO:Al-N) grown by RF magnetron sputtering from an AZO (ZnO with 2 wt% Al2O3) target under nitrogen (N2) atmosphere at room temperature (RT). Nitrogen partial pressures of 0.00, 0.10, 0.25 and 1.00 mTorr were used. The film thickness was around 270 nm. Ultraviolet-Vis-NIR transmittance (T) spectra of the films revealed T values of 80 to 85% in the 400 to 700 nm wavelength range. XRD results indicated that the films had a hexagonal wurtzite structure and were preferentially oriented in the (002) plane. Analyses by EDS indicated that the N atoms tend to be incorporated into the ZnO matrix at the expense of oxygen atoms. The ideal [N]/[Al] was obtained at a N2 partial pressure of 0.25 mTorr, producing a p-type film. For a [N]/[Al] of 1.53, the film also exhibited p-type conduction with an electrical resistivity of 31.92 Ω cm, mobility of 18.65 cm2/V s and carrier density of 1.22 x 1016 cm-3. The low carrier density is attributed to the energetically favorable formation of inactive nitrogen phases instead of acceptor-receiver-acceptor complexes, even at the ideal [N]/[Al].

Synthesis, characterization and UV photodetector application of Sb-doped ZnO nanowires

Sb-doped ZnO nanowires with hexagonal wurtzite structures have been synthesized via chemical vapor deposition method, and the main charge state of Sb dopants is Sb5+. The photoluminescence spectra demonstrate the existence of shallow acceptors, SbZn-2VZn complexes in all probability. On the other hand, Sb doping caused more native and impurity donors in Sb-doped ZnO nanowires. Then, field effect transistors of single Sb-doped ZnO nanowire were fabricated and the transport properties shows n-type conductivity. The competition between shallow acceptors and donors, and the influence of Sb valency need to be investigated further to obtain stable Sb-doped p-type ZnO. At last, we have shown the potential application of Sb-doped ZnO nanowires for nanoscale UV photodetector.

Enhanced bulk photovoltaic effect in ZnO:Bix (p-type)/MoS2 (n-type) heterostructure by tuning Bi content

To investigate the metal oxide thin film application in optoelectronic field via changing its conduction type, the p–n junction with Bi-doped p-type ZnO and n-type MoS2 is fabricated by pulsed laser deposition. In this work, we explore the photovoltaic effect of the ZnO/MoS2 heterostructure, focusing on the function of tuning Bi contents in enhancing the photoelectric conversion efficiency. Doping-Bi ZnO thin film perfectly shows stable p-type semiconductor properties with high carrier concentration (about from 1 × 1016 to 1 × 1017 cm−3), high rational mobility (363 cm−2 V−1 s−1) and low resistivity (216.7 Ω). In addition, MoS2 clearly exhibits large quantities of nanosheets and distinct vertically standing structure. The power conversion efficiency (PCE) of ZnO:Bix/MoS2 enhances from 0.518% to 2.430% along with increasing doping Bi element from 1% to 7%, but when doping Bi content reaches 10%, the PCE decrease 0.552%. Essentially, the ability of capturing photon-generated carriers in ZnO:Bix film gradually increases as doping Bi content to without excessive doping content. These findings illustrate that this proposed Bi-doped ZnO has potential in promoting the photoelectric conversion efficiency of MoS2 based p–n junction.

First-principles study on electronic and optical properties of S, N single-doped and S-N co-doped ZnO

The electronic and optical properties of undoped, N single-doped, S single-doped, and S-N co-doped ZnO were systematically investigated by first-principles calculations. The lattice parameters of S single-doped and S-N co-doped ZnO clearly increased. After N-doping, a strongly localized impurity energy level of N was formed near the Fermi level at the top of valence band (VB). In S-N co-doped ZnO, the localization of N weakened, and the Fermi level went deeper into the VB, indicating that the acceptor energy level of N formed in S-N co-doped system became shallower due to the effect of 3p state of S. Therefore, S-N co-doping is beneficial to obtain p-type ZnO with a higher hole concentration than N single-doping. Compared with undoped ZnO, the static dielectric constant, absorption coefficient, refractive index, energy loss function, and reflectivity of N single-doped, S single-doped, and S-N co-doped ZnO exhibited an increase in low-energy area.

Dual acceptor (N, Cu) doping effects on the electronic and optical properties of ZnO

This study reports on the dual-acceptor doping effects on ZnO for obtaining p-type conductivity and effectively tuning the band gap. The electronic and optical properties of undoped, mono-(N, Cu) and dual-(N-Cu) doped ZnO have been investigated using first-principles calculations based on the density functional theory (DFT) in the framework of generalized gradient approximation (GGA). The substitution of N, Cu and N-Cu into ZnO leads to the acceptor levels arise above the valance band insight the p-type conduction nature of ZnO. The electronic and optical properties revealed that the band gap narrowing of ZnO is possible by these elementals doping. However, the heavy dual doping is an effective way leads to create shallower impurity level below Fermi level. Therefore, the dual-doped ZnO can be a potential candidate for stable p-type ZnO for future experiment and optoelectronic devices applications.

Metal-organic frameworks derived ZnO@MoS2nanosheets core/shell heterojunctions for ppb-level acetone detection: Ultra-fast response and recovery

It is imperative to explore an accurate ppb-level acetone sensor for noninvasive detection of diabetes. In this work, two dimensional p-type MoS2 nanosheets are introduced on the surface of p-type ZnO derived from metal-organic frameworks (MOFs) to produce ZnO@MoS2 core/shell heterojunctions as a novel acetone sensor, showing a great enhancement of acetone response, about two orders of magnitude than that of pure ZnO derived from MOFs. For example, the ZnO@MoS2 exhibits about 80 times enhancement in response to 100 ppb acetone than that of pure ZnO. More importantly, this ZnO@MoS2 heterojunctions sensor exhibits an ultra-fast response/recovery to ultra-low concentration acetone (60 s/40 s @ 5 ppb), which is the best acetone sensing performance for the metal oxide-based materials reported to date. Moreover, the acetone sensing performances are negligibly affected by humidity and other gas, which is suitable for exhaled acetone detection. Finally, it is elucidated that the sharp increase of negative heterojunction interface resistance, ultra-fast gas diffusion rates in MoS2 nanosheets and strong interaction energy are key factors for the excellent acetone sensing properties of ZnO@MoS2. This work opens up a novel and efficient way for ultra-low concentration gas detection.

Homogeneous ZnO nanowire arrays p-n junction for blue light-emitting diode applications.

ZnO is a promising short-wavelength light-emitting materials for its wide bandgap (3.37 eV) and large exciton binding energy (∼60 meV), however, practical p-type doped ZnO is the main challenge in this field. Here, Blue light-emitting diodes (LEDs) based on the homogeneous junctions of Sb doped ZnO nanowire arrays grown on Ga doped ZnO single crystal substrate are fabricated. Element analysis, FET and Hall-effect measurements demonstrate that the Sb atom has been successfully doped into ZnO nanowires to from p-type conductivity. On the benefit of high quality of nano-size homojunction, the fabricated LED shows low turn-on voltage turn-on voltage as low as 3.4 V and strong blue emission peak located at 425 nm at room temperature, which originate from interfacial recombination of ZnO nanowire p-n homojunctions. The present blue LED based on ZnO material may have potential applications in short-wavelength optoelectronic devices.

A Facile and Flexible Approach for Large-Scale Fabrication of ZnO Nanowire Film and Its Photocatalytic Applications.

A novel strategy for large-scale synthesis of ZnO nanowire film is reported, which inherits the advantages of the solution-phase method and seeded growth process, such as low-temperature, efficient, economical, facile and flexible. It is easy to implement on various metals through room-temperature electrodeposition, followed by hydrothermal treatment at 90 °C, and suitable for industrialized production. The ZnO nanowires with an average wire diameter about 40 nm are in situ grown from and on nanocrystalline zinc coating, which forms a strong metallurgical bonding with the substrates. The p-type ZnO nanowire film has a well-preferred orientation along the (100) direction and a wurtzite structure, thereby displaying an effective photocatalytic capability for carcinogenic Cr6+ ions and CO2 greenhouse gas reduction under visible light irradiation. In addition to these features, the ZnO nanowire film is easy to recycle and, therefore, it has broad application prospects in contaminant degradation and renewable energy.